1. Field of Industrial Application
The present invention relates to a new permselective membrane for treating water and organic substances. In particular, the present invention relates to a membrane for separating and concentrating an organic mixture or a water/organic substance mixture by ultrafiltration (UF), microfiltration (MF), pervaporation (PV) or vapor permeation (VP).
2. Prior Art
In the separation of various aqueous solutions, organic liquid mixtures and vapor mixtures, a reverse osmosis membrane, ultrafiltration membrane, dialysis membrane or dehumidifying membrane is practically used. Recently, pervaporation and vapor permeation have attracted attention as new methods for separating organic substances without being influenced by osmotic pressure.
Further, membranes which were used only for separating aqueous substances and inorganic gases in the prior art are now used also for, separating organic solvents and vapors. As the solvent-resistant separating membranes capable of separating such an organic mixture, Teflon microfiltration membranes and polyamide ultrafiltration membranes (Japanese Patent Laid-Open Nos. 71785/1979 and 14908/1983) are known.
Celluloses such as cellulose acetate and aromatic polymers such as polyamides, polysulfones and polyimides are under investigation as the raw materials of pervaporation and vapor permeation membranes used typically for separating a water/alcohol mixture.
Pervaporation membranes heretofore reported for separating a water/acetic acid mixture include a copolymer membrane of acrylic acid and acrylonitrile and that of acrylic acid and styrene, "Maku (Membrane)", 10, 247 (1985); an ion-crosslinked membrane of a blend of polyacrylic acid and nylon 6, J. Appl. Polym. Sci., 35, 119 (1988); an ion-exchange membrane, "Maku (Membrane)" 13, 109 (1988)] and a membrane of a blend of polyvinyl alcohol and a hydrophilic vinyl polymer, Makromol. Chem., 188, 1973 (1987). However, the durabilities of these membranes have not yet been studied and, in addition, the separation characteristics of these membranes are not yet satisfactory.
Membranes comprising polyparabanic acid are disclosed in Japanese Patent Publication No. 46323/1983 and Japanese Patent Laid-Open Nos. 45319/1987, 91123/1988, 91124/1988 and 127029/1989. Polyparabanic acid is described in U.S. Pat. No. 3,661,859 and Japanese Patent Publication No. 19715/1972. Further, Japanese Patent Laid-Open No. 129052/1983 discloses a crosslinking composition containing polybarabanic acid and an organic sulfonic acid or its derivative as a crosslinking agent.
Membranes for separating organic substances, typified by the above-described pervaporation and vapor permeation membranes, must be resistant to not only a high operation temperature but also organic substances to be separated.
A membrane of a polysaccharide having an anionic group (Japanese Patent Laid-Open No. 129104/1985) and a crosslinked polyvinyl alcohol membrane (Japanese Patent Laid-Open No. 109204/1984) which exhibit a high separation capacity in the pervaporation separation of a water/ethanol mixture are unsuitable for the separation of various organic substances other than water/alcohol mixtures from the viewpoints of heat and solvent resistances.